gasm-sdk/asm/riscv_encode.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package asm

// RISC-V register encoding: maps register names to their 5-bit numbers.
// The Go assembler uses the standard RISC-V ABI naming.

// riscvRegNum returns the 5-bit register number for a RISC-V register name.
// Returns -1 if the register is not recognized.
func riscvRegNum(name string) int {
	switch name {
	// Numbered integer registers.
	case "X0", "ZERO":
		return 0
	case "X1", "RA", "LR":
		return 1
	case "X2", "SP":
		return 2
	case "X3", "GP":
		return 3
	case "X4", "TP":
		return 4
	case "X5", "T0":
		return 5
	case "X6", "T1":
		return 6
	case "X7", "T2":
		return 7
	case "X8", "S0", "FP":
		return 8
	case "X9", "S1":
		return 9
	case "X10", "A0":
		return 10
	case "X11", "A1":
		return 11
	case "X12", "A2":
		return 12
	case "X13", "A3":
		return 13
	case "X14", "A4":
		return 14
	case "X15", "A5":
		return 15
	case "X16", "A6":
		return 16
	case "X17", "A7":
		return 17
	case "X18", "S2":
		return 18
	case "X19", "S3":
		return 19
	case "X20", "S4":
		return 20
	case "X21", "S5":
		return 21
	case "X22", "S6":
		return 22
	case "X23", "S7":
		return 23
	case "X24", "S8":
		return 24
	case "X25", "S9":
		return 25
	case "X26", "S10":
		return 26
	case "X27", "S11":
		return 27
	case "X28", "T3":
		return 28
	case "X29", "T4":
		return 29
	case "X30", "T5":
		return 30
	case "X31", "T6", "TMP":
		return 31
	// Floating-point registers (F0-F31).
	case "F0", "FT0":
		return 0
	case "F1", "FT1":
		return 1
	case "F2", "FT2":
		return 2
	case "F3", "FT3":
		return 3
	case "F4", "FT4":
		return 4
	case "F5", "FT5":
		return 5
	case "F6", "FT6":
		return 6
	case "F7", "FT7":
		return 7
	case "F8", "FS0":
		return 8
	case "F9", "FS1":
		return 9
	case "F10", "FA0":
		return 10
	case "F11", "FA1":
		return 11
	case "F12", "FA2":
		return 12
	case "F13", "FA3":
		return 13
	case "F14", "FA4":
		return 14
	case "F15", "FA5":
		return 15
	case "F16", "FA6":
		return 16
	case "F17", "FA7":
		return 17
	case "F18", "FS2":
		return 18
	case "F19", "FS3":
		return 19
	case "F20", "FS4":
		return 20
	case "F21", "FS5":
		return 21
	case "F22", "FS6":
		return 22
	case "F23", "FS7":
		return 23
	case "F24", "FS8":
		return 24
	case "F25", "FS9":
		return 25
	case "F26", "FS10":
		return 26
	case "F27", "FS11":
		return 27
	case "F28", "FT8":
		return 28
	case "F29", "FT9":
		return 29
	case "F30", "FT10":
		return 30
	case "F31", "FT11":
		return 31
	default:
		return -1
	}
}

// RISC-V instruction encoding parameters.
type riscvEnc struct {
	opcode uint32 // bits [6:0]
	funct3 uint32 // bits [14:12]
	funct7 uint32 // bits [31:25]
}

// riscvInstrTable maps RISC-V mnemonics to their encoding.
var riscvInstrTable = map[string]riscvEnc{
	// RV64I, R-type arithmetic/logic.
	"ADD":  {0x33, 0x0, 0x00},
	"SUB":  {0x33, 0x0, 0x20},
	"SLL":  {0x33, 0x1, 0x00},
	"SLT":  {0x33, 0x2, 0x00},
	"SLTU": {0x33, 0x3, 0x00},
	"XOR":  {0x33, 0x4, 0x00},
	"SRL":  {0x33, 0x5, 0x00},
	"SRA":  {0x33, 0x5, 0x20},
	"OR":   {0x33, 0x6, 0x00},
	"AND":  {0x33, 0x7, 0x00},
	// RV64I, 32-bit variants (W suffix).
	"ADDW": {0x3B, 0x0, 0x00},
	"SUBW": {0x3B, 0x0, 0x20},
	"SLLW": {0x3B, 0x1, 0x00},
	"SRLW": {0x3B, 0x5, 0x00},
	"SRAW": {0x3B, 0x5, 0x20},
	// RV64I, I-type shift-immediate (shamt in rs2 field).
	"SLLI":  {0x13, 0x1, 0x00},
	"SRLI":  {0x13, 0x5, 0x00},
	"SRAI":  {0x13, 0x5, 0x20},
	"SLLIW": {0x1B, 0x1, 0x00},
	"SRLIW": {0x1B, 0x5, 0x00},
	"SRAIW": {0x1B, 0x5, 0x20},
	// RV64M, multiply/divide.
	"MUL":    {0x33, 0x0, 0x01},
	"MULH":   {0x33, 0x1, 0x01},
	"MULHSU": {0x33, 0x2, 0x01},
	"MULHU":  {0x33, 0x3, 0x01},
	"DIV":    {0x33, 0x4, 0x01},
	"DIVU":   {0x33, 0x5, 0x01},
	"REM":    {0x33, 0x6, 0x01},
	"REMU":   {0x33, 0x7, 0x01},
	// RV64M, 32-bit variants.
	"MULW":  {0x3B, 0x0, 0x01},
	"DIVW":  {0x3B, 0x4, 0x01},
	"DIVUW": {0x3B, 0x5, 0x01},
	"REMW":  {0x3B, 0x6, 0x01},
	"REMUW": {0x3B, 0x7, 0x01},
	// RV64I, I-type arithmetic.
	"ADDI":  {0x13, 0x0, 0x00},
	"ADDIW": {0x1B, 0x0, 0x00},
	"SLTI":  {0x13, 0x2, 0x00},
	"SLTIU": {0x13, 0x3, 0x00},
	"XORI":  {0x13, 0x4, 0x00},
	"ORI":   {0x13, 0x6, 0x00},
	"ANDI":  {0x13, 0x7, 0x00},
	// Loads (I-type).
	"LB":  {0x03, 0x0, 0x00},
	"LH":  {0x03, 0x1, 0x00},
	"LW":  {0x03, 0x2, 0x00},
	"LD":  {0x03, 0x3, 0x00},
	"LBU": {0x03, 0x4, 0x00},
	"LHU": {0x03, 0x5, 0x00},
	"LWU": {0x03, 0x6, 0x00},
	// Stores (S-type).
	"SB": {0x23, 0x0, 0x00},
	"SH": {0x23, 0x1, 0x00},
	"SW": {0x23, 0x2, 0x00},
	"SD": {0x23, 0x3, 0x00},
	// Branches (B-type).
	"BEQ":  {0x63, 0x0, 0x00},
	"BNE":  {0x63, 0x1, 0x00},
	"BLT":  {0x63, 0x4, 0x00},
	"BGE":  {0x63, 0x5, 0x00},
	"BLTU": {0x63, 0x6, 0x00},
	"BGEU": {0x63, 0x7, 0x00},
	// U-type.
	"LUI":   {0x37, 0x0, 0x00},
	"AUIPC": {0x17, 0x0, 0x00},
	// System.
	"ECALL":  {0x73, 0x0, 0x00},
	"EBREAK": {0x73, 0x0, 0x00},
	"FENCE":  {0x0F, 0x0, 0x00},
	// JALR, indirect jump/call (I-type).
	"JALR": {0x67, 0x0, 0x00},

	// RV64A, atomics (AMO opcode 0x2F).
	// funct3: 0x2 = word, 0x3 = doubleword. funct5 in bits [31:27].
	"AMOSWAPW": {0x2F, 0x2, 0x01 << 2},
	"AMOSWAPD": {0x2F, 0x3, 0x01 << 2},
	"AMOADDW":  {0x2F, 0x2, 0x00 << 2},
	"AMOADDD":  {0x2F, 0x3, 0x00 << 2},
	"AMOANDW":  {0x2F, 0x2, 0x0C << 2},
	"AMOANDD":  {0x2F, 0x3, 0x0C << 2},
	"AMOORW":   {0x2F, 0x2, 0x06 << 2},
	"AMOORD":   {0x2F, 0x3, 0x06 << 2},
	"AMOXORW":  {0x2F, 0x2, 0x04 << 2},
	"AMOXORD":  {0x2F, 0x3, 0x04 << 2},
	"AMOMAXW":  {0x2F, 0x2, 0x14 << 2},
	"AMOMAXD":  {0x2F, 0x3, 0x14 << 2},
	"AMOMINW":  {0x2F, 0x2, 0x10 << 2},
	"AMOMIND":  {0x2F, 0x3, 0x10 << 2},
	"AMOMAXUW": {0x2F, 0x2, 0x1C << 2},
	"AMOMAXUD": {0x2F, 0x3, 0x1C << 2},
	"AMOMINUW": {0x2F, 0x2, 0x18 << 2},
	"AMOMINUD": {0x2F, 0x3, 0x18 << 2},

	// RV64F/D, floating-point arithmetic.
	"FADDS":  {0x53, 0x0, 0x00},
	"FSUBS":  {0x53, 0x0, 0x04},
	"FMULS":  {0x53, 0x0, 0x08},
	"FDIVS":  {0x53, 0x0, 0x0C},
	"FADDD":  {0x53, 0x0, 0x01},
	"FSUBD":  {0x53, 0x0, 0x05},
	"FMULD":  {0x53, 0x0, 0x09},
	"FDIVD":  {0x53, 0x0, 0x0D},
	"FSQRTS": {0x53, 0x0, 0x2C},
	"FSQRTD": {0x53, 0x0, 0x2D},
	// FP loads/stores.
	"FLW": {0x07, 0x2, 0x00},
	"FLD": {0x07, 0x3, 0x00},
	"FSW": {0x27, 0x2, 0x00},
	"FSD": {0x27, 0x3, 0x00},
	// FP min/max.
	"FMINS": {0x53, 0x0, 0x14},
	"FMAXS": {0x53, 0x1, 0x14},
	"FMIND": {0x53, 0x0, 0x15},
	"FMAXD": {0x53, 0x1, 0x15},

	// RV64A, load-reserved / store-conditional (funct5 0x02 / 0x03).
	"LRW": {0x2F, 0x2, 0x02 << 2},
	"LRD": {0x2F, 0x3, 0x02 << 2},
	"SCW": {0x2F, 0x2, 0x03 << 2},
	"SCD": {0x2F, 0x3, 0x03 << 2},

	// FP compare, result in integer register (funct7 0x50/0x51).
	"FEQS": {0x53, 0x2, 0x50},
	"FLTS": {0x53, 0x1, 0x50},
	"FLES": {0x53, 0x0, 0x50},
	"FEQD": {0x53, 0x2, 0x51},
	"FLTD": {0x53, 0x1, 0x51},
	"FLED": {0x53, 0x0, 0x51},
}

// riscvRType encodes an R-type instruction: funct7 | rs2 | rs1 | funct3 | rd | opcode.
func riscvRType(enc riscvEnc, rd, rs1, rs2 int) uint32 {
	return (enc.funct7 << 25) | (uint32(rs2) << 20) | (uint32(rs1) << 15) |
		(enc.funct3 << 12) | (uint32(rd) << 7) | enc.opcode
}

// riscvAMOType encodes an atomic (AMO) instruction.
// Layout: funct5 | aq | rl | rs2 | rs1 | funct3 | rd | opcode.
// The funct5 is stored in the upper bits of enc.funct7 (shifted left by 2).
func riscvAMOType(enc riscvEnc, rd, rs1, rs2 int) uint32 {
	funct5 := enc.funct7 >> 2 // extract funct5 from the stored value
	return (funct5 << 27) | (uint32(rs2) << 20) | (uint32(rs1) << 15) |
		(enc.funct3 << 12) | (uint32(rd) << 7) | enc.opcode
}

// FP conversion instructions (FCVT, FMV).  These use the rs2 field to
// encode the conversion type rather than a register, so they are handled
// separately from the general instruction table.
type riscvCvtEnc struct {
	funct7 uint32 // bits [31:25]
	rs2    uint32 // conversion-type code in bits [24:20]
	opcode uint32 // always 0x53 (OP-FP)
}

var riscvCvtTable = map[string]riscvCvtEnc{
	// float → int (rs2 selects the integer width/sign).
	"FCVTWS":  {0x60, 0x0, 0x53}, // float32 → int32
	"FCVTWUS": {0x60, 0x1, 0x53}, // float32 → uint32
	"FCVTLS":  {0x60, 0x2, 0x53}, // float32 → int64
	"FCVTLUS": {0x60, 0x3, 0x53}, // float32 → uint64
	"FCVTWD":  {0x61, 0x0, 0x53}, // float64 → int32
	"FCVTWUD": {0x61, 0x1, 0x53}, // float64 → uint32
	"FCVTLD":  {0x61, 0x2, 0x53}, // float64 → int64
	"FCVTLUD": {0x61, 0x3, 0x53}, // float64 → uint64
	// int → float (rs2 selects the integer width/sign).
	"FCVTSW":  {0x68, 0x0, 0x53}, // int32 → float32
	"FCVTSWU": {0x68, 0x1, 0x53}, // uint32 → float32
	"FCVTSL":  {0x68, 0x2, 0x53}, // int64 → float32
	"FCVTSLU": {0x68, 0x3, 0x53}, // uint64 → float32
	"FCLASSS": {0x70, 0x0, 0x53}, // classify float32 → GPR mask
	"FCLASSD": {0x70, 0x0, 0x53}, // classify float64 → GPR mask
	"FCVTDW":  {0x69, 0x0, 0x53}, // int32 → float64
	"FCVTDWU": {0x69, 0x1, 0x53}, // uint32 → float64
	"FCVTDL":  {0x69, 0x2, 0x53}, // int64 → float64
	"FCVTDLU": {0x69, 0x3, 0x53}, // uint64 → float64
	// float → float width conversion.
	"FCVTSD": {0x20, 0x1, 0x53}, // float64 → float32
	"FCVTDS": {0x21, 0x0, 0x53}, // float32 → float64
	// Bit moves between integer and FP registers (no conversion).
	"FMVXD": {0x71, 0x0, 0x53}, // float64 → int64 (bit move)
	"FMVDX": {0x79, 0x0, 0x53}, // int64 → float64 (bit move)
	"FMVXW": {0x70, 0x0, 0x53}, // float32 → int32 (bit move)
	"FMVWX": {0x78, 0x0, 0x53}, // int32 → float32 (bit move)
}

// riscvCvtType encodes an FP conversion instruction.
// Layout: funct7 | rs2(convtype) | rs1 | funct3(0) | rd | opcode.
func riscvCvtType(enc riscvCvtEnc, rd, rs1 int) uint32 {
	return (enc.funct7 << 25) | (enc.rs2 << 20) | (uint32(rs1) << 15) |
		(uint32(rd) << 7) | enc.opcode
}

// R4-type fused multiply-add instructions (FMADD/FMSUB/FNMSUB/FNMADD).
// These take 4 register operands: rs1, rs2, rs3, rd.
// Layout: rs3 | fmt | rs2 | rs1 | rm | rd | opcode.
type riscvFmaEnc struct {
	fmt    uint32 // bits [26:25]: 0x0 = single, 0x1 = double
	opcode uint32 // bits [6:0]
}

var riscvFmaTable = map[string]riscvFmaEnc{
	"FMADDS":  {0x0, 0x43}, // rd = rs1*rs2 + rs3
	"FMADDD":  {0x1, 0x43},
	"FMSUBS":  {0x0, 0x47}, // rd = rs1*rs2 - rs3
	"FMSUBD":  {0x1, 0x47},
	"FNMSUBS": {0x0, 0x4B}, // rd = -(rs1*rs2) + rs3
	"FNMSUBD": {0x1, 0x4B},
	"FNMADDS": {0x0, 0x4F}, // rd = -(rs1*rs2) - rs3
	"FNMADDD": {0x1, 0x4F},
}

// riscvFmaType encodes an R4-type fused multiply-add instruction.
func riscvFmaType(enc riscvFmaEnc, rd, rs1, rs2, rs3 int) uint32 {
	return (uint32(rs3) << 27) | (enc.fmt << 25) | (uint32(rs2) << 20) |
		(uint32(rs1) << 15) | (0x0 << 12) /* rm=dynamic */ | (uint32(rd) << 7) | enc.opcode
}

// CSR (Control and Status Register) instructions.
// Format: csr[11:0] | rs1/zimm | funct3 | rd | opcode (0x73).
type riscvCsrEnc struct {
	funct3 uint32 // bits [14:12]
	imm    bool   // true for CSRRWI/CSRRSI/CSRRCI (5-bit uimm variant)
}

var riscvCsrTable = map[string]riscvCsrEnc{
	"CSRRW":  {0x1, false}, // rd=CSR, CSR=rs1
	"CSRRS":  {0x2, false}, // rd=CSR, CSR |= rs1
	"CSRRC":  {0x3, false}, // rd=CSR, CSR &= ~rs1
	"CSRRWI": {0x5, true},  // rd=CSR, CSR=uimm
	"CSRRSI": {0x6, true},  // rd=CSR, CSR |= uimm
	"CSRRCI": {0x7, true},  // rd=CSR, CSR &= ~uimm
}

// riscvCsrType encodes a CSR instruction.
// csr is the 12-bit CSR address; src is either a register number or a 5-bit
// unsigned immediate (depending on enc.imm).
func riscvCsrType(enc riscvCsrEnc, rd, src int, csr int32) uint32 {
	return (uint32(csr&0xFFF) << 20) | (uint32(src&0x1F) << 15) |
		(enc.funct3 << 12) | (uint32(rd) << 7) | 0x73
}

// riscvIType encodes an I-type instruction: imm[11:0] | rs1 | funct3 | rd | opcode.
func riscvIType(enc riscvEnc, rd, rs1 int, imm int32) uint32 {
	return (uint32(imm&0xFFF) << 20) | (uint32(rs1) << 15) |
		(enc.funct3 << 12) | (uint32(rd) << 7) | enc.opcode
}

// riscvSType encodes an S-type instruction: imm[11:5] | rs2 | rs1 | funct3 | imm[4:0] | opcode.
func riscvSType(enc riscvEnc, rs1, rs2 int, imm int32) uint32 {
	immU := uint32(imm) & 0xFFF
	return ((immU >> 5) << 25) | (uint32(rs2) << 20) | (uint32(rs1) << 15) |
		(enc.funct3 << 12) | ((immU & 0x1F) << 7) | enc.opcode
}

// riscvBType encodes a B-type instruction (branches).
func riscvBType(enc riscvEnc, rs1, rs2 int, offset int32) uint32 {
	imm := uint32(offset) & 0x1FFE     // bits [12:1], bit 0 is always 0
	return (((imm >> 12) & 1) << 31) | // imm[12]
		(((imm >> 5) & 0x3F) << 25) | // imm[10:5]
		(uint32(rs2) << 20) | (uint32(rs1) << 15) |
		(enc.funct3 << 12) |
		(((imm >> 1) & 0xF) << 8) | // imm[4:1]
		(((imm >> 11) & 1) << 7) | // imm[11]
		enc.opcode
}

// riscvUType encodes a U-type instruction: imm[31:12] | rd | opcode.
func riscvUType(enc riscvEnc, rd int, imm int32) uint32 {
	return (uint32(imm) & 0xFFFFF000) | (uint32(rd) << 7) | enc.opcode
}

// riscvJType encodes a J-type instruction (JAL).
func riscvJType(rd int, offset int32) uint32 {
	imm := uint32(offset) & 0x1FFFFE   // bits [20:1]
	return (((imm >> 20) & 1) << 31) | // imm[20]
		(((imm >> 1) & 0x3FF) << 21) | // imm[10:1]
		(((imm >> 11) & 1) << 20) | // imm[11]
		(((imm >> 12) & 0xFF) << 12) | // imm[19:12]
		(uint32(rd) << 7) |
		0x6F // JAL opcode
}

// ---- RVC (compressed) encoding helpers ----

// isRVCIntReg reports whether a register number can be encoded in the 3-bit
// prime register field used by compressed instructions (x8-x15).
func isRVCIntReg(r int) bool { return r >= 8 && r <= 15 }

// rvcReg3 returns the 3-bit encoding for registers x8-x15 (0-7).
func rvcReg3(r int) uint32 { return uint32(r - 8) }

// rvcCR encodes a CR-type (register) compressed instruction.
// Format: funct4 | rd/rs1 | rs2 | op=2.
func rvcCR(funct4, rd, rs2 uint32) uint16 {
	return uint16((funct4 << 12) | (rd << 7) | (rs2 << 2) | 0x2)
}

// rvcCI encodes a CI-type (immediate) compressed instruction.
// Used for C.ADDI, C.LI, C.LUI, C.ADDIW, linear 6-bit immediate.
func rvcCI(funct3, rd uint32, imm uint32) uint16 {
	return uint16((funct3 << 13) | ((imm>>5)&1)<<12 | (rd << 7) | (imm&0x1F)<<2 | 0x1)
}

// rvcSLLI encodes C.SLLI, which shares funct3=0 with C.ADDI but lives in the
// op=10 quadrant (unlike C.ADDI's op=01).
func rvcSLLI(rd, shamt uint32) uint16 {
	return uint16(((shamt>>5)&1)<<12 | (rd << 7) | (shamt&0x1F)<<2 | 0x2)
}

// encodeRVCPattern extracts the bits listed in pattern (MSB first) from imm
// into a packed value, matching cmd/internal/obj/riscv's encodeBitPattern.
func encodeRVCPattern(imm uint32, pattern []int) uint32 {
	packed := uint32(0)
	for _, bit := range pattern {
		packed = packed<<1 | (imm>>bit)&1
	}
	return packed
}

// rvcLSP encodes a stack-relative compressed load (op=10 quadrant): C.LWSP
// (funct3=2, 4-byte scale), C.LDSP (funct3=3) or C.FLDSP (funct3=1, 8-byte
// scale).  offset is the full byte offset.
func rvcLSP(funct3, rd uint32, offset uint32) uint16 {
	pattern := []int{5, 4, 3, 8, 7, 6}
	if funct3 == 0x2 {
		pattern = []int{5, 4, 3, 2, 7, 6}
	}
	packed := uint32(0)
	for i, b := range pattern {
		packed |= ((offset >> b) & 1) << (5 - i)
	}
	return uint16((funct3 << 13) | ((packed>>5)&1)<<12 | (rd << 7) | (packed&0x1F)<<2 | 0x2)
}

// rvcSSP encodes a stack-relative compressed store (op=10 quadrant): C.SWSP
// (funct3=6, 4-byte scale), C.SDSP (funct3=7) or C.FSDSP (funct3=5, 8-byte
// scale).  offset is the full byte offset.
func rvcSSP(funct3, rs2 uint32, offset uint32) uint16 {
	pattern := []int{5, 4, 3, 8, 7, 6}
	if funct3 == 0x6 {
		pattern = []int{5, 4, 3, 2, 7, 6}
	}
	packed := uint32(0)
	for i, b := range pattern {
		packed |= ((offset >> b) & 1) << (5 - i)
	}
	return uint16((funct3 << 13) | (packed << 7) | (rs2 << 2) | 0x2)
}

// rvcCL encodes a register-relative compressed load (op=00 quadrant): C.LW
// (funct3=2), C.LD (funct3=3) or C.FLD (funct3=1).  imm is the full byte
// offset; the immediate bits are extracted per the RISC-V CL format.
func rvcCL(funct3, rd, rs1 uint32, imm uint32) uint16 {
	pattern := []int{5, 4, 3, 7, 6}
	if funct3 == 0x2 {
		pattern = []int{5, 4, 3, 2, 6}
	}
	packed := encodeRVCPattern(imm, pattern)
	return uint16((funct3 << 13) | ((packed>>2)&0x7)<<10 | (rs1 << 7) | ((packed & 0x3) << 5) | (rd << 2))
}

// rvcCS encodes a register-relative compressed store (op=00 quadrant): C.SW
// (funct3=6), C.SD (funct3=7) or C.FSD (funct3=5).  imm is the full byte
// offset; the immediate bits are extracted per the RISC-V CS format.
func rvcCS(funct3, rs2, rs1 uint32, imm uint32) uint16 {
	pattern := []int{5, 3, 7, 6}
	if funct3 == 0x6 {
		pattern = []int{5, 3, 2, 6}
	}
	packed := encodeRVCPattern(imm, pattern)
	return uint16((funct3 << 13) | ((packed>>2)&0x7)<<10 | (rs1 << 7) | ((packed & 0x3) << 5) | (rs2 << 2))
}

// rvcCIW encodes a CIW-type compressed immediate wide instruction: C.ADDI4SPN
// (funct3=0).  imm is the raw byte offset.
func rvcCIW(funct3, rd uint32, imm uint32) uint16 {
	packed := encodeRVCPattern(imm, []int{5, 4, 9, 8, 7, 6, 2, 3})
	return uint16((funct3 << 13) | (packed << 5) | (rd << 2))
}

// rvcCA encodes a CA-type (arithmetic) compressed instruction.
// Format: funct6[15:10] | rd'/rs1'[9:7] | funct2[6:5] | rs2'[4:2] | op=01.
func rvcCA(funct6, funct2, rd, rs2 uint32) uint16 {
	return uint16((funct6 << 10) | (rd << 7) | (funct2 << 5) | (rs2 << 2) | 0x1)
}

// rvcCBShift encodes a CB-type shift/immediate compressed instruction
// (C.SRLI, C.SRAI, C.ANDI).  rd is the 3-bit prime-register index; imm is
// the 6-bit shamt/immediate; funct2 selects the operation (0=SRLI, 1=SRAI,
// 2=ANDI).
func rvcCBShift(funct2, rd, imm uint32) uint16 {
	return uint16((0x4 << 13) | ((imm>>5)&1)<<12 | (funct2 << 10) | (rd << 7) | (imm&0x1F)<<2 | 0x1)
}

// rvcADDI16SP encodes C.ADDI16SP: ADDI rd, imm, rd for the stack pointer
// with a 10-bit signed, 16-byte-scaled immediate.  imm is the raw byte
// offset; the immediate bits are extracted in the order [9|4|6|8:7|5].
func rvcADDI16SP(rd uint32, imm int32) uint16 {
	u := uint32(imm)
	packed := uint32(0)
	for _, bit := range []uint{9, 4, 6, 8, 7, 5} {
		packed = packed<<1 | (u>>bit)&1
	}
	return uint16((0x3 << 13) | ((packed>>5)&1)<<12 | (rd << 7) | (packed&0x1F)<<2 | 0x1)
}